I haven't made my mind up about the padding yet. I usually wear hard shell elbow pads, exactly as you say, for crawling around, but I'm not sure if the UBACS padding is enough for the points of my elbows and, although I haven't tried it yet to be certain, I can't imagine that I could fit my usual pads over the top.
I do like the idea of having padding on my forearms too though - crawling over/through brambles is fine in the "heat of battle" (lol - u kno wot i mean) but I quite often go at it with more gusto than my adrenalin level will carry me through without flinching. I wish the Viper UBACS had velcro or something so you could take the padding out easily and I can see myself adding that feature, because the perfect solution will end up being cutting the padding into shape to allow hard shell elbow pads to be worn at the same time...